At RIMarkable, one of Rob's readers reports some insider-y news that this coming fall, SprintNextel will release a new BlackBerry that will bridge Sprint and Nextel's Push-to-Talk Networks.

Keep in mind that despite the merger of these two companies a year and a half ago, their two PTT's are walled off: Nextel is iDEN and Sprint's is CDMA.
Qualcomm's QChat is said to be the solution to solve this.

QChat's main web page (linked below) says the solution enables communication to begin with the press of a Push to Talk button on the handset, as opposed to a standard cellular call. It forms a call by combining separate point-to-point connections between each IP endpoint at a managing entity known as the QChat Applications Server, deployed on the carrier's IP Wide Area Network (WAN.)

"Pressing the Push to Talk button originates a call to the target QChat user and provides the originator with information indicating the availability (presence) of the target user," QChat's website points out. "If the target user is available, the originator receives an immediate indication that the target user is available and the originator can begin speaking. The call originator's voice is then sent through the carrier's network to the target's handset. Initiating a call to a QChat user who is not available will simply result in a negative response tone rather than a busy signal or a voicemail."